Lithographic apparatus, level sensor, method of inspection, device manufacturing method, and device manufactured thereby

ABSTRACT

A level sensor for a lithographic projection apparatus, the level sensor comprising a light source, a first reflector, a second reflector and a detector the first reflector being arranged to direct light from the light source towards a wafer surface, and the second reflector being arranged to direct light reflected from the wafer surface to the detector, wherein the first and second reflectors are configured to incur a minimal process dependent apparent surface depression.

The present invention relates to a level sensor for a lithographic apparatus. Lithographic projection apparatus are well known in the prior art, and generally comprise:

-   -   a radiation system for supplying a projection beam of radiation;     -   a support structure for supporting patterning means, the         patterning means serving to pattern the projection beam         according to a desired pattern;     -   a substrate table for holding a substrate; and     -   a projection system for projecting the patterned beam onto a         target portion of the substrate.

The term “patterning means” as here employed should be broadly interpreted as referring to means that can be used to endow an incoming radiation beam with a patterned cross-section, corresponding to a pattern that is to be created in a target portion of the substrate; the term “light valve” can also be used in this context. Generally, the said pattern will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit or other device (see below). Examples of such patterning means include:

-   -   A mask. The concept of a mask is well known in lithography, and         it includes mask types such as binary, alternating phase-shift,         and attenuated phase-shift, as well as various hybrid mask         types. Placement of such a mask in the radiation beam causes         selective transmission (in the case of a transmissive mask) or         reflection (in the case of a reflective mask) of the radiation         impinging on the mask, according to the pattern on the mask. In         the case of a mask, the support structure will generally be a         mask table, which ensures that the mask can be held at a desired         position in the incoming radiation beam, and that it can be         moved relative to die beam if so desired;     -   A programmable mirror array, One example of such a device is a         matrix-addressable surface having a viscoelastic control layer         and a reflective surface. The basic principle behind such an         apparatus is that (for example) addressed areas of the         reflective surface reflect incident light as diffracted light,         whereas unaddressed areas reflect incident light as undiffracted         light, Using an appropriate filter, the said undiffracted light         can be filtered out of the reflected beam, leaving only the         diffracted light behind; in this manner, the beam becomes         patterned according to the addressing pattern of the         matrix-addressable surface. An alternative embodiment of a         programmable mirror array employs a matrix arrangement of tiny         mirrors, each of which can be individually tilted about an axis         by applying a suitable localized electric field, or by employing         piezoelectric actuation means. Once again, the mirrors are         matrix-addressable, such that addressed mirrors will reflect an         incoming radiation beam in a different direction, to unaddressed         mirrors; in this manner, the reflected beam is patterned         according to the addressing pattern of the matrix-addressable         mirrors. The required matrix addressing can be performed using         suitable electronic means. In both of the situations described         hereabove, the patterning means can comprise one or more         programmable mirror arrays. More information on mirror arrays as         here referred to can be gleaned, for example, from United States         patents U.S. Pat. No. 5,296,891 and U.S. Pat. No. 5,523,193, and         PCT patent applications WO 98/38597 and WO 98/33096, which are         incorporated herein by reference. In the case of a programmable         mirror array, the said support structure may be embodied as a         frame or table, for example, which may be fixed or movable as         required; and     -   A programmable LCD array. An example of such a construction is         given in United States patent U.S. Pat. No. 5,229,872, which is         incorporated herein by reference. As above, the support         structure in this case may be embodied as a frame or table, for         example, which may be fixed or movable as required.

For purposes, of simplicity, the rest of this text may, at certain locations, specifically direct itself to examples involving a mask and mask table; however, the general principles discussed in such instances should be seen in the broader context of the patterning means as hereabove set forth.

Lithographic projection apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, the patterning means may generate a Circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising one or more dies) on a substrate (silicon wafer) that has been coated with a layer of radiation-sensitive material (resist). In general, a single wafer will contain a whole network of adjacent target portions that are successively irradiated via the projection system, one at a time. In current apparatus, employing patterning by a mask on a mask table, a distinction can be made between two different types of machine. In one type of lithographic projection apparatus, each target portion is irradiated by exposing the entire mask pattern onto the target portion in one go; such an apparatus is commonly referred to as a wafer stepper or step-and-repeat apparatus. In an alternative apparatus—commonly referred to as a step-and-scan apparatus—each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction (the “scanning” direction) while synchronously scanning the substrate table parallel or anti-parallel to this direction; since, in general, the projection system will have a magnification factor M (generally <1), the speed V at which the substrate table is scanned will be a factor M times that at which the mask table is scanned. More information with regard to lithographic devices as here described can be gleaned, for example, from U.S. Pat. No. 6,046,792, incorporated herein by reference.

In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate (also referred to as a wafer) that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure, bake (PEB), development, a hard bake and measurement/inspection of the imaged features. This array of procedures is used as a basis to pattern an individual layer of a device, e.g. an IC. Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, will have to be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, incorporated herein by reference.

For the sake of simplicity, the projection system may hereinafter be referred to as the “lens”; however, this term should be broadly interpreted as encompassing various types of projection system, including refractive optics, reflective optics, and catadioptric systems, for example. The radiation system may also include components operating according to any of these design types for directing, shaping or controlling the projection beam of radiation, and such components may also be referred to below, collectively or singularly, as a “lens”. Further, the lithographic apparatus may be of a type having two or more substrate tables (and/or two or more mask tables). In such “multiple stage” devices the additional tables may be used in parallel, or preparatory steps may be carried out on one or more tables Awhile one or more other tables are being used for exposures. Dual stage lithographic apparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791, both incorporated herein by reference.

The projection lens has a large numerical aperture, and consequently has a narrow focus plane. When imaging a pattern onto a substrate it is necessary to ensure that an uppermost surface of the substrate (i.e. the surface onto which the pattern is to be imaged) lies within the focus plane of the projection lens. A level sensor is used determine the position of the uppermost surface of the substrate. If the level sensor indicates that the uppermost surface of the substrate does not lie in the focus plane of the projection lens, the height of the substrate table bearing the substrate is adjusted accordingly.

The level sensor is also used to measure any slope that may be present in the surface of the substrate. The substrate table is tilted accordingly.

The level sensor is an important part of a lithographic projection apparatus. As the wavelengths used by lithographic apparatus become shorter, the focus depth of the projection lens reduces, and the accuracy of the level sensor becomes increasingly important.

Level sensors used for lithographic projection apparatus are subject to process dependency. Process dependency is a form of error in which level sensor measurements provide differing results depending on how a substrate being measured has previously been processed. For example, a level sensor may provide a particular height measurement for a wafer comprising a silicon substrate coated with a single layer of resist, and may provide a different height measurement for a wafer comprising a silicon substrate coated with several layers of resist, even if both wafers are at the same actual height. The error is referred to as process dependent apparent surface depression, and is understood to be caused by an optical effect known as the Goos-Haenchen shift [ref: F. Goos and H. Haenchen, Ann. Phys. 1 (6), 333 (1947)]. The Goos-Haenchen shift is a lateral translation of light along a reflecting surface (in this case the resist) during reflection. The shift is dependent upon the material and layer structure of the substrate, and in addition is wavelength and polarization dependent. As a result of apparent surface depression a substrate may not be correctly located in the focus plane of the projection lens. When this occurs the resolution of a pattern imaged onto the substrate will be compromised.

Process dependent apparent surface depression is by its nature very difficult to monitor. Although some calibration for process dependency may be performed, residual process dependent apparent surface depression will remain. Different processes give rise to widely varying apparent surface depression, so that a different calibration is needed for each process. In addition, different lithographic apparatus experience widely varying apparent surface depression in response to the same processes (i.e. a given process may give rise to a particular apparent surface depression in one lithographic apparatus, and may give rise to an entirely different apparent surface depression in a different lithographic apparatus). This means that a calibration performed for one lithographic apparatus cannot be used for another lithographic apparatus.

It is an object of the present invention to substantially overcome or mitigate at least one of the above problems.

According to a first aspect of the invention there is provided a level sensor for a lithographic projection apparatus, the level sensor comprising a light source, a first reflector, a second reflector and a detector, the first reflector being arranged to direct light from the light source towards a wafer surface, and the second reflector being arranged to direct light reflected from the wafer surface to the detector, wherein the first and second reflectors are configured to incur a minimal process dependent apparent surface depression.

The term ‘light’ is not limited to electromagnetic radiation in the visible spectrum, but is intended to cover electromagnetic radiation of any suitable wavelength.

The term ‘configuration’ is not limited to inherent physical properties of the mirrors, and may include the positions and orientations of the mirrors.

The term ‘minimal’ is not intended to limit the invention to the case where the process dependent apparent surface depression is substantially zero. Instead, it is intended to mean that for a given reflector type which gives rise to a range of possible apparent surface depressions, the configuration of the reflectors is such that the apparent surface depression lies within a low portion of the range.

The invention has arisen from the inventors' realization that process dependency of the apparent surface depression is not caused solely by the substrate surface, but is also caused by mirrors of the level sensor (a realization that is counterintuitive since the mirrors are not modified in any way by wafer processing).

The minimal apparent surface depression may be a net minimum of the apparent surface depression of the combination of the first and second mirrors.

Alternatively, the minimal apparent surface depression may be a net minmum of the apparent surface depression of the combination of the first and second mirrors together with the wafer surface.

According to a second aspect of the invention there is provided a plurality of lithographic projection apparatus comprising:

-   -   a radiation system for providing a projection beam of radiation;     -   a support structure for supporting patterning means, the         patterning means serving to pattern the projection beam         according to a desired pattern;     -   a substrate table for holding a substrate; and     -   a projection system for projecting the patterned beam onto a         target portion of the substrate,         wherein each lithographic apparatus further comprises a level         sensor comprising a light source, a first reflector, a second         reflector and a detector, the first reflector being arranged to         direct light from the light source towards a wafer surface, and         the second reflector being arranged to direct light reflected         from the wafer surface to the detector, wherein the         configuration of the first and second reflectors for each level         sensor is substantially identical, thereby substantially         eliminating relative variation of process dependent apparent         surface depression between each lithographic apparatus.

The first and second reflectors may be mirrors.

The mirrors may comprise Ag coated with Al2O3.

The thickness of the coating is preferably in the range 150 nm to 300 nm, and most preferably in the range 150 nm to 200 nm.

The reflectors may be arranged in pairs, with a configuration that provides substantially zero apparent surface depression for each pair.

Each pair of reflectors may comprise two surfaces of a prism or pentaprism. Alternatively, mirrors may be used.

The mirrors of the level sensor may be dielectric mirrors.

Although specific reference may be made in this text to the use of the lithographic apparatus according to the invention in the manufacture of ICs, it should be explicitly understood that such an apparatus has many other possible applications. For example, it may be employed in the manufacture of integrated optical systems, guidance and detection patterns for magnetic domain memories, liquid-crystal display panels, thin-film magnetic heads, etc. The skilled artisan will appreciate that, in the context of such alternative applications, any use of the terms “reticle”, “wafer” or “die” in this text should be considered as, being replaced by the more general terms “mask”, “substrate” and “target portion”, respectively.

In the present document, the terms “radiation” and “beam” are used to encompass all types of electromagnetic radiation, including ultraviolet (UV) radiation (e.g. with a wavelength of 365, 248, 193, 157 or 126 nm) and extreme ultra-violet (EUV) radiation (e.g. having a wavelength in the range 5-20 nm), as well as particle beams, such as ion beams or electron beams.

Embodiments of the invention will now be described, by way of example only, with reference to die accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

FIG. 1 depicts a lithographic projection apparatus including a level sensor;

FIG. 2 depicts a level sensor to which the invention may be applied;

FIG. 3 depicts a level sensor and an associated problem identified by the inventors and addressed by the invention;

FIG. 4 depicts an alternative level sensor to which the invention may be applied;

FIG. 5 depicts part of a level sensor which embodies the invention;

FIG. 6 depicts a level sensor which embodies the invention; and

FIGS. 7 and 8 are graphs which show results obtained using models which are used to construct embodiments of the invention.

FIG. 1 schematically depicts a lithographic projection, apparatus 1 according to a particular embodiment of the invention. The apparatus comprises:

-   -   a radiation system Ex, IL, for supplying a projection beam PB of         radiation (e.g. UV radiation). In this particular case, the         radiation system also comprises a radiation source LA;     -   a first object table (mask table) MT provided with a mask holder         for holding a mask MA (e.g. a reticle), and connected to first         positioning means (not shown) for accurately positioning the         mask with respect to item PL;     -   a second object table (substrate table) WT provided with a         substrate holder for holding a substrate W (e.g. a resist-coated         silicon wafer), and connected to second positioning means (not         shown) for accurately positioning the substrate with respect to         item PL; and     -   a projection system (“lens”) PL (e.g. a quartz and/or Ca₂ lens         system or a catadioptric system comprising lens elements made         from such materials, or a mirror system) for imaging an         irradiated portion of the mask MA onto a target portion C (e.g.         comprising one or more dies) of the substrate W.

As here depicted, the apparatus is of a transmissive type (i.e. has a transmissive mask). However, in general, it may also be of a reflective type, for example (with a reflective mask). Alternatively, the apparatus may employ another kind of patterning means, such as a programmable mirror array of a type as referred to above.

The source LA (e.g. an ArF laser) produces a beam of radiation (e.g. at 193 nm). This beam is fed into an illumination system (illuminator) IL, either directly or after having traversed conditioning means, such as a beam expander Ex, for example. The illuminator IL may comprise adjusting means AM for setting the outer and/or inner radial extent (commonly referred to as σ-outer and σ-inner, respectively) of the intensity distribution in the beam. In: addition, it will generally comprise various other components, such as all integrator IN and a condenser CO. In this way, the beam PB impinging on the mask MA has a desired uniformity and intensity distribution in its cross-section.

It should be noted with regard to FIG. 1 that the source LA may be within the housing of the lithographic projection apparatus (as is often the case when the source LA is a mercury lamp, for example), but that it may also be remote from the lithographic projection apparatus, the radiation beam which it produces being led into the apparatus (e.g. with the aid of suitable directing mirrors); this latter scenario is often the case when the source LA is an excimer laser. The current invention and claims encompass both of these scenarios. The beam PB subsequently intercepts the mask MA, which is held on a mask table MT. Having traversed the mask MA, the beam PB passes through the lens PL, which focuses the beam PB onto a target portion C of the substrate W. With the aid of the second positioning means (and interferometric measuring means IF), the substrate table WT can be moved accurately, e.g. so as to position different target portions C in die path of the beam PB. Similarly, the first positioning means can be used to accurately position the mask MA with respect to the path of the beam PB, e.g. after mechanical retrieval of the mask MA from a mask library, or during a scan. In general, movement of the object tables MT, WT will be realized with the aid of a long-stroke module (coarse positioning) and a short-stroke module (fine positioning), which are not explicitly depicted in FIG. 1. However, in the case of a wafer stepper (as opposed to a step-and-scan apparatus) the mask table MT may just be connected to a short stroke actuator, or may be fixed. Mask MA and substrate W may be aligned using mask alignment marks M1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in two different modes:

1. In step mode, die mask table MT is kept essentially stationary, and an entire mask image is projected in one go (i.e. a single “flash”) onto a target portion C. The substrate table WT is then shifted in the x and/or y directions so that a different target portion C can be irradiated by the beam PB; and

2. In scan mode, essentially the same scenario applies, except that a given target portion C is not exposed in a single “flash”. Instead, the mask table MT is movable in a given direction, (the so-called “scan direction”, e.g. the y direction) with a speed ν, so that the projection beam PB is caused to scan over a mask image; concurrently, the substrate table, WT is simultaneously moved in the same or opposite direction at a speed V=Mv, in which M is the magnification of the lens PL (typically, M=¼ or ⅕). In this manner, a relatively large target portion C can be exposed, without having to compromise on resolution.

A level sensor, generally indicated as LS in FIG. 1, is used to measure the height and slope of the uppermost surface of the substrate W. The wafer table WT is raised or lowered, and tilted, accordingly to ensure that the uppermost surface of the substrate W is located in the focus plane of the projection lens PL.

FIG. 2 illustrates a level sensor to which the invention may be applied. A beam of light 1 generated by a suitable source S is directed via a folding mirror 2 to a substrate 3. The beam of light 1 is reflected from the surface of the substrate and is directed via a second folding mirror 4 to a detector 5.

A change of height of the surface of the substrate 3 is seen as a displacement of the location at which the beam of light 1 is incident upon the detector 5. For example, if the surface of the substrate is flat then the beam of light 1 is incident upon the center of the detector 5. If die surface of the substrate includes an indentation, shown by dotted line 3 a, then the point at which the beam of light is reflected is displaced, as shown by broken line 1 a. An associated translation of the beam of light 1 a occurs at the detector 5, indicating that the height of the surface of the substrate has changed. The height of the surface of the substrate 3 is thus closely monitored by the level sensor.

Apparent surface depression arises because the beam of light 1 in general is not entirely directly reflected from the upper surface of the substrate 3. Instead, a portion of the beam of light 1 is translated along the surface of the substrate 3 for a short distance before being reflected (this is known as the Goos-Haenchen shift). Typically, die distance of translation is of the order of one wavelength of the beam of light.

If the distance of the translation were to be consistent then it would be possible to calibrate the level sensor to account for the apparent surface depression. However, it has been found that the apparent surface depression has a strong dependency upon the structure of the surface of the substrate. For example, a bare silicon wafer will give rise to an apparent surface depression, whereas a substrate comprising a silicon wafer coated with photo-resist will give rise to an entirely different apparent surface depression. A substrate comprising a wafer coated with several layers of resist will give rise to a particularly large apparent surface depression.

The process dependency of the apparent surface depression is reduced by ensuring that the beam of light 1 has a broad wavelength band and has a variety of polarizations. The invention provides a further reduction of the process dependency of the apparent surface depression. This is achieved via the selection of the folding mirrors 2, 4 as described below.

The inventors have realized that the folding mirrors 2, 4 of the level sensor are a primary source of process dependency. This would, at first sight appear not to be possible, since the folding mirrors 2, 4 do not change in any way when a process is applied to a substrate, but instead always remain the same. Indeed, it would seem to be the case that process dependency must arise entirely from die substrate since the substrate is the only article to which the process has been applied. However, the inventors have realized that the mirrors give rise to an apparent surface depression which is wavelength and polarization dependent, and that this wavelength and polarization dependency is added to the apparent surface depression generated by the substrate surface to give a combined apparent surface depression which will vary from lithography apparatus to lithography apparatus and will vary from process to process. Furthermore, the inventors have realized that appropriate selection of the mirrors will reduce this variation.

A simplified example of, the combined effect of the mirrors and the substrate surface is shown in FIG. 3. Referring to FIG. 3 a, a light source for a level sensor provides a beam 1 which includes p-polarized light and s-polarized light. Folding mirror 2 has a polarization dependent Goos-Haenchen shift which causes p-polarized light to be translated (the s-polarized light is not translated). Upon reflection from the folding mirror 2 the beam is separated into two: a translated p-polarized beam as represented by broken line 1 c, and an untranslated s-polarized beam as represented by the dotted line 1 d. The surface of the substrate 3 reflects the p-polarized beam with an additional slight translation (the s-polarized beam is not translated). The detector 5 detects the p-polarized and s-polarized beams as a single beam with a center point 6 (assuming that p-polarized and s-polarized beams are of equal intensity). The detected height includes an apparent surface depression which is caused in part by the mirror 2 and in part by the substrate 3.

FIG. 3 b shows the effect of using the same level sensor to detect a substrate 3 having a different surface. The mirror 2 has the same wavelength dependent Goos-Haenchen shift, but the substrate provides a much larger shift, with the result that the center point 6 of the detected beam is shifted to the left. When two level sensors are used to measure the substrate of FIGS. 3 a and 3 b, each level sensor will suffer a different apparent surface depression error, due to differences between the mirrors of the level sensors.

The above example is overly simplified for the purposes of illustration. There are several different aspects which increase the complexity of the apparent surface depression. When the substrate 3 comprises a ‘wafer stack’ (i.e. a wafer coated with several layers of resist) it acts as a polarization filter, preferentially reflecting s-polarized light. This means that the intensities of the p-polarized beam 1 c and the s-polarized beam 1 d are not equal at the detector 5, causing a shift of the detected center point 6. In addition, the mirror 4 will give rise to a polarization dependent Goos-Haenchen shift.

As indicated above, the beam 1 is not a single wavelength but comprises a broad band of wavelengths. The broad band is used because it has been found in the past to reduce apparent surface depression (via averaging of the wavelength dependent Goos-Haenchen shift). A wavelength dependent Goos-Haenchen shift arises at each of the folding mirrors 2,4 and at the surface of the substrate 3. In addition, the folding mirrors 2,4 and the substrate 3 may act as wavelength filters.

The polarization dependent and wavelength dependent Goos-Haenchen shifts, together with the polarization dependent and wavelength dependent filtering, provided by the folding mirrors 2, 4 and the substrate 3 give rise to an apparent surface depression which has an extremely complicated process dependency.

The apparent surface depression has been found to vary from process to process (i.e. from processed substrate to processed substrate) and from level sensor to level sensor. It had not previously been realized until now that the mirrors provide a significant contribution to the process dependent apparent surface depression.

In addition to realizing that the mirrors provide an important contribution to the apparent surface depression, the inventors have realized that this contribution may be, mitigated by controlling those parameters of die mirrors which give rise to the Goos-Haenchen shift via the selection of appropriate mirrors.

The selection of appropriate mirrors has a twofold benefit. The mirrors may be selected to provide a level sensor which has a reduced apparent surface depression (for a variety of processed substrates), thereby providing improved level sensing for a given level sensor. In addition, where several lithography apparatus are used, machine to machine variation of the process dependent apparent surface depression can be substantially eliminated by ensuring that the mirrors of the level sensors in the lithography apparatus all have the same properties (i.e. all give rise to the same Goos-Haenchen shift).

Thus, in an embodiment of one aspect of the invention, level sensors are provided for a multiplicity of lithography apparatus, the mirrors of the levels sensors having substantially identical optical properties. The mirrors may be metallic, for example Ag, and may be provided with a coating, for example Al₂O₃. The coating has been found by the inventors to give rise to a Goos-Haenchen shift, the wavelength and polarization dependency of the shift being dependent upon the thickness of the coating. Therefore in this embodiment of the invention the thickness of die coating of the mirrors used for the level sensor is controlled to provide a minimized Goos-Haenchen shift (appropriate thicknesses are described further below). A ‘family’ of level sensors (and the lithographic apparatus to which they are fitted) may be defined for which the mirrors of the level sensors are substantially identical. For example, it may be specified that the mirrors are all fabricated together in a single batch. This would provide the advantage set out below.

Eliminating machine to machine variation of process dependent apparent surface depression is particularly advantageous. Currently, a calibration of apparent surface depression may be performed for a lithography apparatus for a substrate to which a particular process (process A) has been applied. The calibration is used for that lithography apparatus each time a substrate is used to which process A has been applied. The calibration is time consuming and costly. When using prior art level sensors, a level sensor of a second lithography apparatus would need to be calibrated separately for process A, and similarly for third, fourth lithography apparatus, etc. Where the first aspect of the invention is applied, i.e. where the mirrors of the level sensors have substantially identical properties, only one level sensor must be calibrated for process A, and the resulting calibration is used for all of the level sensors (i.e. for all of the lithographic apparatus to which the level sensors are fitted). This provides a significant time and cost saving. Similarly, for a substrate to which a different process, process B, has been applied, one level sensor is calibrated and die resulting calibration is used for all of the level sensors (i.e. for all of the lithographic apparatus to which the level sensors are fitted).

In addition to ensuring that the mirrors of different level sensors have substantially identical properties, it is advantageous to provide mirrors which give rise to minimal apparent surface depression. It may appear that this is not necessary, given that a calibration may be performed for each process, which may be used for many level sensors. However, the calibration does not eliminate apparent surface depression. For example, consider a substrate having semiconductor regions and copper regions. Each of these will give rise to a different apparent surface depression, so that although an, average apparent surface depression may be calibrated, specific regions of the substrate will still cause the level sensor to suffer different apparent surface depressions. This is further complicated by the fact that there is often substantial variation between substrates that are nominally identical.

An embodiment of a second aspect of the invention comprises a level sensor provided with mirrors having optical properties which are selected to minimize process dependent apparent surface depression. For example, the second aspect of the invention may be embodied in a level sensor having metallic mirrors, Referring to FIG. 2, the level sensor comprises a source S, a detector 5, and folding mirrors 2, 4 comprising an Ag substrate with a Al₂O₃ coating. The coating has been found by the inventors to give rise to a Goos-Haenchen shift, the wavelength and polarization dependency of the shift being dependent upon the thickness of the coating. Therefore in this embodiment of the invention the thickness of the coating of the mirrors used for the level sensor is selected to provide a minimized Goos-Haenchen shift. The appropriate thickness is dependent to some extent upon the processes that have been applied to the substrate 3. However, it has been found that in general a coating of between 150 nm and 300 nm (preferably between 150 nm and 200 nm) provides favorable results.

In addition to being applicable to level sensors of the form shown in FIG. 2, the first and second aspects of the invention may be applied to other forms of level sensor. For example, FIG. 4 shows an alternative level sensor to which the invention may be applied. Referring to FIG. 4, a level sensor comprises a source S, detector 5 and four folding mirrors 2 a, 2 b, 4 a, 4 b. It can be seen that the level sensor shown in FIG. 4 corresponds to that shown in FIG. 2, except that each folding mirror 2, 4 is replaced with a pair of folding mirrors 2 a, 2 b, and 4 a, 4 b.

It will be appreciated that the invention is not restricted to level sensors which use metallic folding mirrors, but may be applied when any form of beam-steering optics is used. The beam-steering optics may for example comprise a prism arranged to allow a beam to enter the prism, and then undergo internal reflection from a face of the prism before leaving the prism at a different angle. The prism may be provided with a coating on the external side of the face which provides the reflection, the coating being controlled according to the invention to provide a minimal Goos-Haenchen shift. Additionally or alternatively, the angle of incidence of the beam into the prism may be adjusted to reduce the Goos-Haenchen shift arising at the prism.

In an alternative embodiment of the invention the metallic mirrors are replaced with fully dielectric mirrors, i.e. mirrors comprising a non-conducting medium. Dielectric mirrors have a zero Goos-Haenchen, effect, and so do not contribute to apparent surface depression. Apparent surface depression only occurs for reflecting surfaces that cause a phase jump of reflected light which is dependent on the angle of incidence. In mathematical terms apparent surface depression only occurs when the following condition is satisfied: ${\frac{\partial\phi}{\partial\theta_{i}} \neq 0},$ where φ is die phase jump (the argument of the complex reflection coefficient) and θ₁ is the angle of incidence of the light beam. For a boundary between air and a dielectric medium (with a purely real refractive index) the derivative is zero. A disadvantage of using dielectric mirrors is that they preferentially reflect s-polarized light, with the result that apparent surface depression caused by the surface of the wafer is accentuated (the averaging effect provided by using all polarizations is lost). An advantage of using dielectric mirrors is that, since the apparent surface depression is caused entirely by the surface of the wafer, machine to machine variations of the apparent surface depression are substantially eliminated.

The dielectric mirror used by the embodiment of the invention comprises a silicon slab which has an anti-reflective coating on its reverse side. The reflection loss caused by the silicon is at an acceptably low level. Dielectric mirrors may be implemented using any suitable material. The embodiment may for example be of the type shown in FIG. 2 or the type shown in FIG. 4.

In a further alternative embodiment of the invention, each folding mirror of the level sensor may be replaced with a pair of metallic mirrors. Each pair of metallic mirrors provides a zero net Goos-Haenchen shift, because the mirrors are arranged so that the second mirror provides a Goos-Haenchen shift which is opposite to that provided by the first mirror. An example of a suitable pair of mirrors is shown in FIG. 5 b. Referring to FIG. 5 b, a pair of identical metallic mirrors 21, 22 are symmetrically arranged so that a beam of light 23 is incident upon both mirrors with the same angle of incidence. Since the metallic mirrors 21, 22 are identical, including having the same thickness of coating layer, the Goos-Haenchen shift caused by each mirror cancels out to give a net apparent surface depression of zero. This is effective for every wavelength and every polarization state, due to the symmetry of the mirror arrangement.

The effect of the mirrors 21, 22 can be understood by comparing them with a single mirror, as shown in FIG. 5 a. Referring to FIG. 5 a, a beam 24 is incident upon a mirror 25. P-polarized light of the beam is translated by the mirror 25 upon reflection, whereas s-polarized light of the beam is not. This gives rise to an s-polarized beam 24 a and a p-polarized beam 24 b which are spatially separated. In FIG. 5 b, the Goos-Haenchen shift at mirror 22 gives rise to an s-polarized beam 23 a and a p-polarized beam 23 b. However, due to the symmetry of the mirror configuration, an equal and opposite Goos-Haenchen shift occurs at mirror 21, so that the beams 23 a, 23 b are recombined to form a single beam 23.

It will be appreciated that there are other configurations of optics which make it possible to use two reflections to substantially eliminate the Goos-Haenchen shift. For example, two internal reflections inside a suitably dimensioned reflecting prism may be used (tile prism may have any suitable number of sides).

A disadvantage of this embodiment of the invention, and of that shown in FIG. 5, is that it occupies extra space as compared to the embodiment of the invention shown in FIG. 2. In a further alternative embodiment of die invention a level sensor is constructed using two non-identical metallic mirrors which are chosen such that the Goos-Haenchen shift occurring at the first mirror is at least partially corrected by the Goos-Haenchen shift occurring at the second mirror, thereby leading to a minimal apparent surface depression. This is shown schematically in FIG. 6. A first mirror 32 of the level sensor gives rise to a shift of the reflection of p-polarized light 31 c but not s-polarized light 31 b, whereas a second mirror 34 of the level sensor gives rise to a shift of the reflection of s-polarized light 31 b but not p-polarized light 31 c (for simplicity of illustration a substrate 33 is assumed not to give rise to any shift). The net apparent surface depression caused by the mirrors 32, 34 is zero.

It will be appreciated that in practice there are many variables which complicate the selection of two mirrors to provide a minimal net apparent surface depression (using different mirrors or identical mirrors). Variables which are independent of the mirrors are the angle of incidence of die level sensor beam on the mirrors, the wavelength range of the level sensor beam and the polarization state of the level sensor beam, Variables of the mirrors themselves are the number of coating layers on the mirrors and the thickness of each coating layer.

The inventors have modeled the effect of coating layer thickness for a level sensor comprising two folding mirrors based on a Ag substrate with an Al₂O₃ coating, in a level sensor which uses an unpolarized light beam in the wavelength range 600-1050 nm (generated by a halogen lamp). The angle of incidence of the light on the folding mirrors was 55 degrees. The level sensor performance was modeled in relation to a substrate comprising photoresist on silicon and photoresist on aluminum. The angle of incidence of the light on the substrate was 70 degrees.

For a given pair of mirror coating thicknesses, the apparent surface depression caused by the mirrors was determined for a range of resist thicknesses (200-1000 nm), first for the silicon and photoresist substrate then for the aluminum and photoresist substrate. This gave two sets of apparent surface depression values. A set of difference values was obtained by subtracting the second set from the first set, and a weighted average of the set was obtained. The model did not include apparent surface depression caused by die substrate.

The graphs shown in FIG. 7 illustrate that the apparent surface depression exhibits maxima and minima as a function of mirror coating thickness. In FIG. 7 b a peak (light grey region in FIG. 7 a) is indicative of a large error, whereas a trough (dark grey region in FIG. 7 a) is indicative of a small error. It can been seen from FIG. 7 that all minima are found for non-identical coating thickness combinations. Although the data shown in FIG. 7 is complex it can be seen that in general terms a reasonably low error will be achieved provided that one of the mirrors is provided with a coating that is around 150 nm (+/−50 nm) thicker or thinner than the coating of die other mirror. A non-zero error arises when both mirrors are provided with zero thickness (i.e. no) coatings; this is because the metal of the mirrors gives rise to a Goos-Haenchen shift.

This model indicates mirror-coating thicknesses which will give rise to a minimal apparent surface depression, assuming that no apparent surface depression is caused by the substrate. In practice it is known that the substrate will always give rise to some apparent surface depression, and so this model is of limited utility. The model might be useful to determine appropriate mirror-coating thicknesses for a level sensor to be fitted in a lithographic apparatus which will be used for a wide variety of processed substrates (since it may not be possible to model the apparent surface depression caused by the substrates). In some instances it may be known in advance that a lithographic apparatus will be used for a particular given processed substrate. Where this is the case a model may be used to determine the thickness of mirror coatings which provides a minimal apparent surface depression, talking into account the apparent surface depression caused by the substrate.

It will be appreciated that this represents a significant step as compared to above embodiments. Instead of minimizing or eliminating the apparent surface depression arising from mirrors, the apparent surface depression arising from the mirrors is used to counteract the apparent surface depression occurring on the substrate (i.e. the net total apparent surface depression is minimized).

The range of solutions for optimum coating stack parameters is large. The optimum choice depends on the angle of incidence of the level sensor beam on the mirrors, the wavelength range and polarization state of the level sensor beam, the number of coating layers on the mirrors and the thickness of each coating layer. Additional variables are the angle of incidence of the level sensor beam on the substrate, and the expected combination of materials on the substrate.

The inventors have constructed a second model, for a level sensor having two folding mirrors based on a Ag substrate with an Al₂O₃ coating, a level sensor beam comprising unpolarized light in the wavelength range 600-1050 nm, and a substrate comprising photoresist on silicon and photoresist on aluminum. The angle of incidence of the light on the folding mirrors was 55 degrees. The angle of incidence of the light on the substrate was 70 degrees.

The apparent surface depression effect of mirrors and substrate was calculated averaged over a resist thickness range 200-600 nm (the model operated in the same manner as the model described further above, except that the apparent surface depression effect of the substrate was included in the model), FIG. 8 b shows the effect of varying the coating thickness of the mirrors, peals indicate large errors and troughs indicating small errors (this is shown two-dimensionally in FIG. 8 a). It can be seen from FIG. 8 that a minimal error occurs in a region centered approximately on coating thicknesses of 200 nm for both mirrors. In addition, a low error occurs in regions where one mirror has a coating thickness of approximately 200 nm and the other has a coating thickness of approximately 520 nm.

The second model was determined for a particular substrate, i.e. photoresist-on-silicon with a thickness range of 200 nm-600 nm. Where it is known that a lithographic apparatus (and the level sensor fitted to it) are to be used, the model may be used to select appropriate mirrors for the level sensor, for example with coatings 200 nm thick. Where it is known that a lithographic apparatus (and level sensor) are to be used for a different substrate, the model may be modified to determine appropriate mirrors for the level sensor. In this way the mirrors may be tailored to be optimal for those processed substrates for which it is intended to use a given lithographic apparatus.

Generally it has been found that mirror thicknesses in the range 150 nm to 300 nm provide minimal apparent surface depression. For the reasons mentioned above, consistency between level sensor of different lithographic apparatus of a ‘family’ is important; a particular value in the range 150 nm to 200 nm may be selected and used for the level sensors in all of the lithographic apparatus of the family.

Whilst specific embodiments of the invention have been described above, it will be appreciated that the invention may be practiced otherwise than as described. The description is not intended to limit the invention. 

1. A plurality of lithographic projection apparatus comprising: a radiation system configured to provide a projection beam of radiation; a support structure configured to support a patterning structure, the patterning structure serving to pattern the projection beam according to a desired pattern; a substrate table configured to hold a substrate; and a projection system configured to project the patterned beam onto a target portion of the substrate, wherein each lithographic apparatus includes a level sensor comprising: a first reflector positioned to direct light from a light source towards a wafer surface; and a second reflector positioned to direct light reflected from the wafer surface to a detector, wherein the first reflectors of the level sensors of the plurality of lithographic apparatus are substantially identical to one another, and wherein the second reflectors of the level sensors of the plurality of lithographic apparatus are substantially identical to one another.
 2. The plurality of lithographic projection apparatus according to claim 1, wherein a process dependent apparent surface depression of one of the plurality of lithographic apparatus is substantially equal to a process dependent apparent surface depression of another of the plurality of lithographic apparatus.
 3. The plurality of lithographic projection apparatus according to claim 1, wherein each of the first and second reflectors of the level sensors of the plurality of lithographic apparatus comprises a mirror.
 4. The plurality of lithographic apparatus according to claim 3, wherein a reflective surface of the mirror of each of at least one of the first and second reflectors comprises gold.
 5. The plurality of lithographic apparatus according to claim 3, wherein a reflective surface of the mirror of each of at least one of the first and second reflectors comprises a dielectric material.
 6. The plurality of lithographic projection apparatus according to claim 3, wherein each of said mirrors is metallic, and wherein a reflective surface of the mirror of each of at least one of the first and second reflectors has a transparent coating.
 7. The plurality of lithographic apparatus according to claim 6, wherein a reflective surface of the mirror of each of at least one of the first and second reflectors comprises at least one of silver and aluminum.
 8. The plurality of lithographic projection apparatus according to claim 6, wherein said coating comprises at least one of aluminum oxide, magnesium fluoride, and silicon oxide.
 9. A lithographic apparatus comprising a level sensor, said level sensor comprising: a first reflector configured to direct a beam from a light source toward a wafer surface; and a second reflector configured to direct the beam from the wafer surface to a detector, wherein a reflective surface of the first reflector has a coating, said coating having an optical thickness of 430 nanometers +/−60 nanometers at a central wavelength of the beam, and wherein a reflective surface of the second reflector has a coating, said coating having an optical thickness of 430 nanometers +/−60 nanometers at a central wavelength of the beam.
 10. The lithographic apparatus according to claim 9, wherein the coating of a reflective surface of at least one of the first and second reflectors comprises aluminum oxide.
 11. The lithographic apparatus according to claim 9, wherein the coating of a reflective surface of at least one of the first and second reflectors comprises at least one of aluminum oxide, magnesium fluoride, and silicon oxide.
 12. The lithographic apparatus according to claim 9, wherein the coating of a reflective surface of at least one of the first and second reflectors comprises a layer of aluminum oxide having a thickness of 275 nanometers +/−60 nanometers.
 13. The lithographic apparatus according to claim 9, wherein at least one among the first and second reflectors includes a metallic mirror having a transparent coating.
 14. A lithographic apparatus comprising a level sensor, said level sensor comprising: a first reflector configured to direct a beam from a light source toward a wafer surface; and a second reflector configured to direct the beam from the wafer surface to a detector, wherein a reflective surface of the first reflector has a coating, and wherein a reflective surface of the second reflector has a coating, and wherein the optical thickness of the coating of the reflective surface of the first reflector at a central wavelength of the beam differs from the optical thickness of the coating of the reflective surface of the second reflector at a central wavelength of the beam by 205 nanometers +/−60 nanometers.
 15. The lithographic apparatus according to claim 14, wherein the coating of the reflective surface of at least one of the first and second reflectors comprises aluminum oxide.
 16. The lithographic apparatus according to claim 14, wherein the coating of the reflective surface of at least one of the first and second reflectors comprises at least one of aluminum oxide, magnesium fluoride, and silicon oxide.
 17. The lithographic apparatus according to claim 14, wherein the coating of the reflective surface of at least one of the first and second reflectors comprises a layer of aluminum oxide having a thickness of 220 nanometers +/−60 nanometers.
 18. The lithographic apparatus according to claim 14, wherein at least one among the first and second reflectors includes a metallic mirror having a transparent coating.
 19. The lithographic apparatus according to claim 14, wherein the optical thickness of the coating of the reflective surface of one of the first and second reflectors at a central wavelength of the beam is 140 nanometers +/−60 nanometers, and wherein the optical thickness of the coating of the reflective surface of the other of the first and second reflectors at a central wavelength of the beam is 345 nanometers +/−60 nanometers.
 20. The lithographic apparatus according to claim 14, wherein the reflective surface of one of the first and second reflectors has a coating of aluminum oxide having a thickness of 90 nanometers +/−60 nanometers, and wherein the reflective surface of the other of the first and second reflectors has a coating of aluminum oxide having a thickness of 220 nanometers +/−60 nanometers.
 21. A lithographic apparatus comprising a level sensor configured to measure a height of a wafer surface, said level sensor comprising: a first reflector configured to direct a beam from a light source toward the wafer surface; and a second reflector configured to direct the beam from the wafer surface to a detector, wherein a reflective surface of at least one of the first and second reflectors comprises aluminum having a coating of native oxide.
 22. The lithographic apparatus according to claim 21, wherein a magnitude of an apparent depression of the wafer surface due to translation of the beam at reflective surfaces of the first and second reflectors is less than thirty-five nanometers.
 23. The lithographic apparatus according to claim 21, wherein an error in the height of the wafer surface as indicated by a corresponding position of incidence of the beam at the detector is less than seventy nanometers.
 24. The lithographic apparatus according to claim 21, wherein the first reflector includes: a first surface configured to direct the beam toward the wafer surface; and a second surface configured to direct the beam toward the first surface.
 25. The lithographic apparatus according to claim 21, wherein the second reflector includes: a first surface configured to direct the beam to the detector; and a second surface configured to direct the beam toward the first surface.
 26. A lithographic apparatus comprising a level sensor, said level sensor comprising: a first reflector configured to direct a beam from a light source toward a wafer surface; and a second reflector configured to direct the beam from the wafer surface to a detector, wherein the first reflector includes a first surface configured to direct the beam toward the wafer surface and a second surface configured to direct the beam toward the first surface, and wherein the second reflector includes a first surface configured to direct the beam to the detector and a second surface configured to direct the beam toward the first surface.
 27. The lithographic apparatus according to claim 26, wherein an apparent surface depression of the first reflector is substantially zero, and wherein an apparent surface depression of the second reflector is substantially zero.
 28. The lithographic apparatus according to claim 26, wherein at least one among the first and second reflectors comprises a prism having the respective first and second surfaces.
 29. The lithographic apparatus according to claim 26, wherein said prism comprises a pentaprism. 